1. Field of the Invention
This invention relates to a system for controlling an automatic vehicle transmission, more particularly to a system for, and a method of, controlling an automatic vehicle transmission.
2. Description of the Related Art
Prior-art automatic vehicle transmissions are provided with multiple ranges (positions), typically P, R, N, D4, D3, 2 and 1. In the automatic gearshift range D4, gearshift is conducted automatically in accordance with a gearshift scheduling map (characteristics) stored in an on-board microcomputer memory, based on the vehicle speed and the throttle opening (indicative of the engine load) as parameters indicating the vehicle operating conditions. The vehicle driver ordinarily drives with the shift lever setting in the D4 range. This prior-art automatic gearshift mode is hereinafter referred to as "auto-shift mode" or "auto-shifting means" in the specification.
Since in the D4 range in the auto-shift mode, gearshift is conducted independently of any desire on the part of the driver, if the driver desires to down shift for sporty driving or for utilizing the engine braking effect, he must manually move the shift lever from the D4 range to the D3 or 2 range. On the other hand, when he again desires to upshift from the gear, he must return the shift lever to the D4 range. This means that the driver is required to frequently move the shift lever when running on a winding or hilly road.
Therefore, as taught by Japanese Laid-Open Patent application Nos. Sho 59(1984)--37359, Hei 2(1990)--8545 and Hei 6(1994)--74318, it has been proposed to provide, in the automatic vehicle transmission, a switch or lever separate of the shift lever which enables the driver to upshift or down shift by manual operation of the switch or lever. The use of such a device will hereinafter be referred to as "manual-shift mode" or "manual-shifting means" in the specification.
In this kind of automatic vehicle transmission having the auto-shift mode and the manual-shift mode, when gearshift was conducted in the auto-shift mode and then switched to manual-shift mode, there may occur a situation that the vehicle driver does not notice or realize the gearshift and generate a manual-shifting command. As a result, the gears are shifted to a gear which is not intended by him, rendering the drivability or driver's comfort degraded due to the engine output decrease. To be more specific, the gearshift may sometimes causes the engine to rev excessively or make the engine braking effect greater than needed, resulting in the degradation of the drivability or driver's comfort. Alternatively, this may lead the engine speed to drop, making the driver feel poor acceleration when the vehicle is accelerated.
Moreover, it has been proposed in U.S. Pat. No. 5,484,350, etc., to provide a automatic vehicle transmission control system, in which a plurality of gearshift scheduling maps such conditions as for level-road running, hill climbing or hill descending. Based on the vehicle acceleration, an index estimating whether the vehicle is running a hilly road is calculated and one from among the maps is selected using the index. Gearshift control in the auto-shift mode is then conducted in accordance with the characteristics defined in the selected map in response to the detected engine speed and engine load. This prior art technique aims to improve the drivability or driver's comfort during running on a hilly road.
When the gearshift control system is based on the technique described in the United Sates patent, since gearshift points are different from the maps and gearshift is conducted using any of the maps, the driver is more likely to overlook it so that he may generate the manual-shifting command, resulting in a down shifting or upshifting to a gear which is not intended, and causing the above-mentioned disadvantages to occur.